Jean-Marc Ayrault said analysis of samples taken at the scene of the April 4 attack in rebel-held Khan Sheikhun in which 31 children were among the dead showed "there is no doubt that sarin gas was used" and that it was produced by Syrian laboratories.
"There is no doubt about the responsibility of the Syrian regime given the way that the sarin used was produced," Ayrault told journalists after the report was presented at a meeting of French defence chiefs.
He said the substance France believes was used in the attack contains hexamine, a component that was also found in a gas attack in northwest Syria in 2013.
"We are able to confirm that the sarin used on April 4 is the same sarin that was used in an attack in Saraqeb on April 29, 2013," he said.
Ayrault said the chemical fingerprint is "typical of the method developed in Syrian laboratories".
"This (production) method bears the regime's hallmarks and allows us to determine its responsibility for this attack," he said.
A French diplomat said the analysis was carried out on unexploded ordnance found at Khan Sheikhun.
- 'Same as 2013 attack' -
Syrian President Bashar al-Assad, backed by his ally Russia, has strongly denied allegations that his forces used chemical weapons against the town, describing it as a "100 percent fabrication".
That agreement was later enshrined in a UN Security Council resolution.
In a policy U-turn, US President Donald Trump ordered air strikes on the Syrian airbase from which Washington believes the attack was launched.
US Defence Secretary Jim Mattis said on Friday there was "no doubt" Syria has retained some chemical weapons and warned Assad's regime not to use them.
"There can be no doubt in the international community's mind that Syria has retained chemical weapons in violation of its agreement and its statement that it had removed them all," Mattis said during a visit to Israel.
Mattis added that the Damascus regime would be "ill-advised to try to use any again", adding: "We've made that very clear with our strike."
On Monday, the US government placed 271 Syrian chemists from the Syrian Scientific Studies and Research Center (SSRC) and other officials on its financial blacklist in response to their presumed role in the chemical weapons attack.
Washington says the SSRC was responsible for developing the alleged sarin gas weapon.
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